TAB technology is commonly used when high lead counts and small package size are desired. When TAB connections are made, an electrically conductive bead (or "bump") connects the die input or output to an electrical lead. The bump acts to provide an electrical connection which brings the effective electrical path of the bond pads above the level of the integrated circuit surface. A parallel technology provides bumps on the lead tape for TAB processing.
In traditional batch bump-forming processes, all the dies on a wafer, whether good or bad, are processed to provide bumps at each possible bond pad. These methods are commonly used when a large number of small-sized chips are present on the wafer. TAB bumps can be formed by batch processes such as evaporation through a mask, or plating using a photoresist pattern. Evaporated bump formation is limited by deposition efficiency: a large portion of the evaporated gold ends up on the mask. Although the gold is recoverable, the additional processing required to retrieve the gold raises the over-all costs. In addition, the resulting bumps have less-than-optimal geometry, and are not necessarily uniform in height. The cost of photoresist medium limits the desirability of photoresist methods. Additionally, photoresist bump formations exhibit low adhesion properties if the surface is not scrupulously cleaned prior to plating.
When a relatively small number of larger dies are present on a wafer, it can be cost effective to form individual ball bumps in position on the good dies, and minimize further processing of the bad dies. Ball bump formation, in which the bump is formed using a Ball Bumper, provides an alternate to traditional TAB bump formation. As shown in FIG. 1, a traditional bond pad/ball bump structure 10 includes a bump 12 of gold metal which is formed on an aluminum pad 14. A passivation layer 16 covers the surface of the integrated circuit die 18.
Prior art ball bump formation generally begins with sorting of the dies on a yielded semiconductor wafer. Dies are tested, good dies are noted, and bad dies are inked. Alternately, the bad dies can be marked using a laser, or computer-mapped. Gold bumps are formed directly on the aluminum bond pad using a Ball Bumping machine to produce a structure such as that shown in FIG. 1. Individual ball bumps are bonded to individual leads (not shown) using a device such as a single point bonder.
Current ball bumping methods and structures exhibit limitations and drawbacks. The bonding between gold and aluminum is not preferred, as it is difficult to produce a strong and durable bond. Insufficient bonding between the gold ball and the aluminum bond pad can produce bond failure. Temperature-dependent bond failures are especially troublesome, as they may be intermittent and difficult to detect.
Ball bump structures of the prior art are subject to corrosion, especially at the gold/aluminum interface, where gold/aluminum intermetallic formations can lead to bond failure. Moisture and other chemicals can intrude into the bond, causing bond failure.
Because of the temperatures and pressures involved with ball bumping, ball bumps of the prior art must be bonded to the lead system individually, which requires significant bonding time. Prior art attempts at mass ball-bumping or mass coining have resulted in chip cracking, unreliable dies, and chip failure.